For example, in a conventional color video printer or the like, the Cy (cyan), M (magenta), and Ye (yellow) signals are mainly processed. FIG. 2 shows an example of the processes typically involved. The A/D converted R, G and B signals are LOG converted into the Cy, M, and Ye signals. Further, these signals are subjected to the masking correction and become Cy', M', and Ye' signals. These signals are input to a head driver and the heads are driven, so that a color image is printed. The masking correction is performed by the following matrix arithmetic operation. ##EQU1##
However, this method has a drawback that the good color reproduction is not obtained. Namely, if the logarithm conversion is simply performed, there is a case in which the concentration becomes a value above 3.0 because of the difference of the dynamic range of the input image signal and the difference of the reproducing range of the ink of the output. In such a case, the concentration obviously exceeds the maximum concentration of the print. On the other hand, a method of correcting the concentration range by the gamma conversion process is used. However, this method has a drawback that the saturation changes and the reproduced color image differs from the original image.
Even in a color display apparatus as well as the foregoing color printer, as shown in FIG. 13, a frame buffer is necessary for each image signal in order to display a color image. In this case, a problem occurs with respect to which kind of image signal is stored in the frame buffer. Namely, in the conventional example of FIG. 13, the R, G and B signals are converted by the matrix conversion into the luminance signal Y and two color difference signals (R-Y and B-Y). The Y, R-Y, and B-Y signals are stored and thereafter, the necessary image processes are performed. For example, in the case of displaying the color image by CRT device (not shown), those signals are again converted into the R, G and B signals and output. The storage of the color image signal in, e.g., the frame buffer or the like causes a problem. If the number of pixels cannot be reduced in the interest of saving the memory capacity, the bit number in the direction of depth of the color image signal cannot help decreasing. However, hitherto, for example, when the number of bits in the depth direction of each of two color difference signals is set to six, there is a problem such a difference occurs between the original image and the reproduced color image because of the decrease in number of bits. To avoid such problem, eight bits are needed to obtain good color reproduction. Due to such situation, since the number of memories cannot be reduced, the memory capacity and the cost increase.
On the other hand, hitherto, two kinds of methods have been generally used in order to obtain good color balance in the image.
(1) The color balance is adjusted before photographing.
(2) The photographed image is corrected.
Method (1) corresponds to "the white balance switch" of a video camera. A white paper or the like is photographed prior to starting the photographing operation and the white balance is set using the "white" image as a reference. Method (2) is widely used in the printing field and the like. However, it largely depends on the feeling and experience of the craftsman.
Therefore, in the case of method (2), hitherto, it is impossible to automatically set the color balance.
In the case of digitizing, hitherto, for example, the least significant bit is omitted or the data is merely compressed a regular interval. In the case of digitizing a regular interval, unless there is a limitation of the capacity of a ROM, the color reproducibility is improved more and more as the interval is made increasingly. However, there is a certain limitation of the ROM capacity as mentioned above.
When the saturation distribution of the color image signal from the actual natural image is examined, it will be understood that it is a rare case that the color image signals are uniformly distributed in color space, and in most cases, the color image signal falls within a region below the half value of the maximum saturation as shown in FIG. 20.
Since it is considered that the influence on the color reproducibility of the whole image by the color of a certain saturation corresponds to the number of pixels having the saturation, the influence of the pixels in the portion having a large saturation with a small distribution is relatively small. Therefore, in the case of constituting the masking ROM by the digitization of the equivalent interval, the portion corresponding to the high saturation in the ROM occupies a constant capacity although it is hardly used for the masking. Therefore, this portion is the vain portion (i.e., not efficiently used). Namely, in order to obtain the high color reproducibility, the portion of a relatively low saturation in which many pixels are distributed must be finely digitized. Thus, the capacity of the ROM increases and the vain degree (size of the vain portion) also increases.